Process for delignification of lignocellulosic materials



Nov. 16, 1965 Filed Dec. 19, 1961 Louls-PHlLlPPE CLERMONT ETAL 3,218,226

PROCESS FOR DELIGNIFICATION OF LIGNOCELLULOSIC MATERIALS 5 Sheets-Sheet 1 Temperature C N0v 16, 1965 .LOUIS-PHILIPPE CLERMONT ETAL 3,218,226

PROCESS FOR DELIG'NIFICATION OF LIGNOCELLULOSIC MATERIALS coonmq Tune Hows Nov. 16, 1965 Per Cen! Lignln in Pulp w 3 oU|s-P|||| |PPE CLERMONT ETAL 3,218,225

PROCESS FOR DELIGNIFICATION 0F LIGNOCELLULOSIC MATERIALS Filed Dec. 19, 1961 uo S7. voler added LEGEND o z 1 Caching Time- Hours 95 Temnerufure C Puln Yield r Per Cen! 111 Nood) 5 Sheets-Sheet S 95 In Tempemluve C Jgd@ United States Patent O 3,2i8,226 PROCESS FOR DELIGNIFCATION OF LIGNOCELLULOSIC MATERIALS louis-Philippe Clermont, Orleans, Ontario, and Frederick Bender, Ottawa, Ontario, Canada, assignors to Canadian Patents and Development Limited, Ottawa, Ontario, Canada, a company Filed Dec. I9, 1961, Ser. No. 160,502 t Claims. (Ci. 162-72) This invention relates to the delignication of lignocellulosic materials.

Separation of the main components of wood is usually considered under two aspects, i.e., the industrially important pulping processes and the fundamentally important isolation of lignin in as undegraded a state as possible, regardless of the carbohydrate fraction.

In the industrial pulping processes, lignin is removed under rather drastic conditions. As long as carbohydrate losses can be kept low and degradation of cellulose beyond a certain point can be avoided, any process based on inexpensive and/Or recoverable chemicals has potential. Degradation of the lignin in the course of the process does not present any problem since the lignin is burned or wasted in almost all cases.

The question whether cellulose and lignin are or are not chemically linked in wood has yet to be settled. If a chemical bond exists, solution of one component would be impossible without breaking this bond. Since we do not know of a solvent for wood, a wood-swelling solvent containing a mild agent capable of breaking the celluloselignin bond and subsequently permitting the freed components to go into solution appears to be a logical approach to the isolation of relatively unchanged lignin and cellulose. By choosing a non-aqueous system, hydrolytic degradation could be kept at a minimum.

For this reason, delignification by means of organic solvents has been of considerable interest. Some solvents which have become available recently show a strong swelling action on wood and even dissolve small quantities of lignin (or lignin fractions) under mild conditions. However, no solvent has been found as yet which would remove the complete lignin fraction from wood.

It is an object of this invention to provide a process for pulping lignocellulosic materials wherein lower cooking temperatures and shortened cooking times may be employed.

Another object is to provide a pulping process wherein the yield is readilly controllable by varying the cooking time and/ or temperature.

Another object is to provide a pulping process wherein delignification may be carried out to an increased degree and wherein the resulting pulp may be easily bleached.

Another object is to provide a process for readily obtaining pulps of high alpha-cellulose and high cupriethylene-diamine viscosities or low copper numbers.

Another object is to provide a pulping process wherein the dissolved lignin may be easily recovered from the cooking liquors by the simple addition of water, and wherein solvent recovery is facilitated.

Other objects and advantages of the invention will become apparent as this description proceeds.

It has theretofore has been proposed to use dimethylsulphoxide (DMSO) as the basis of delignifying agents. However, while DMSO is a powerful solvent and wood- ICC swelling agent, it does not by itself have any eifective pulping action when used in the treatment of wood.

It has now been found that dimethylsulphoxide having dissolved therein a minor percentage of an inorganic gas of the type of nitrogen dioxide, sulphur dioxide, or chlorine, is a most effective and satisfactory delignifying agent.

The invention will be described with reference to the accompanying drawings, in which FIGURES 1 to 17, inclusive, are graphs depicting results of experiments.

In accordance with the invention, wood material in the form of chips or sawdust is subjected to a cooking treatment wherein the cooking liquor consists essentially of dirnethylsulphoxide having dissolved therein approximately 2 to 10% of an inorganic gas selected from the group nitrogen dioxide, (NO2) sulphur dioxide, (SO2) and chlorine, (C12).

A series of experiments have been carried out and the results thereof are plotted in FIGURES 1 to 17, inclusive. The tests were conducted with black spruce and aspen poplar sawdust (20-60 mesh fractions) and aspen chips. The wood used was air dry; this however, is not essential; the moisture content of wood does not interfere with the pulping process. The experiments were carried out using a 10:1 liquor to wood ratio.

FIGURE 1 to 11, inclusive and 13, show the results of experiments conducted with solutions of 5% and 10% SO2 in DMSO and illustrate clearly the following points:

(a) Eect 0f cooking time on pulp yield, lignin content and pentosan content of pulps from black spruce sawdust FIGURE 1 shows that at 140 deg. C., the pulp yield decreases rapidly, until after 3 hours a yield of about 45 percent is obtained with a technical grade DMSO containing 5 percent water. Under more nearly anhydrous conditions, yields are appreciably higher.

FIGURE 2 shows the very pronounced effect of the addition of a small amount of water to the DMSO on the rate of lignin removal. It appears that hydrolytic action increases the speed of delignication. The pH as measured by means of a glass electrode was lower in all cases where water was added. The fact that delignitication also took place when no water was added to the DMSO does not necessarily mean that hydrolytic action is not essential to delignication since DMSO is a very hygroscopic solvent and some amounts of water are probably always present in it and in the wood.

The lignins recovered 4from cooks with 5 percent and l0 percent SO2 in DMSO, with and without the addition of water, were :analyzed for sulphur after thorough washing with distilled Water. None of these lignins contained sulphur. This result indicates that the delignication mechanism simply consists in hydrolysis followed by solution of the hydrolyzed lignin. It also suggests the posibility of SO2 recovery.

FIGURE 3 shows that pentosans are removed at essentially the same rate whether the reaction is carried out under anhydrous conditions or not. Lowering the pH by water addition does not appreciably increase the speed of removal of pentosans.

(b) Eect of cooking temperalure on pulp yield, lignin content and pentosan content of pulps from black spruce sawdust FIGURE 4 indicates that not much pulping action takes place below deg. C. By increasing the temperature from 120 deg. to 140 deg. C., pulp yields may be obtained ranging from about 8O percent down to 48 percent.

FIGURE 5 shows that rapid delignication takes place after a temperature of about 125 deg. C. has been reached. Below 125 deg. C. very little lignin is removed by a water containing 5 percent SO2 solution after 3 hours of cooking, whereas with the more anhydrous solution, a slight increase in the residual lignin is 'observed under the same conditions. That this increase in lignin is not due to greater pentosan removal may be seen from FIGURE 6, which shows once more that pentosans are removed at about the same rate.

(c) Eect of water content of DMSO on pulp yield, Lignin content and pentosun content of pulps from black spruce sawdust FIGURE 7 shows that the pulp yield goes through a minimum at about 2() percent water content in the pulping liquor. As Imay be seen from a comparison of the two curves, a ra-pid delignication takes place during the first hour as indicated by a decrease in pulp yield from 63 to. 55 percent when the water content is increased from 5 to 20 percent. There is a marked levelling off after 3 hours lcooking when the yield has decreased to -about 43 percent for pulps cooked with DMSO solutions containing from 5 to 20 percent water.

FIGURE 8 shows results which parallel those shown in FIGURE 7. As the water content in the DMSO is increased up to about 20 percent the residual lignin in the pulp decreases to a minimum. With further addition of water, delignication becomes 4more and more diflicult due to increasingly acidic conditions which promote lignin condensation reactions rather than delignitication reactions.

FIGURE 9 indicates that increasing the water content of the DMSO from O to 50 lpercent has little effect on residual pentosans.

(d) Efect of SO2 concentration in DMSO on lignin content of pulp and pulp yield from black spruce sawdust A comparison of FIG. 10 and FIG. 2 shows that increasing the SO2 concentration from 5 to l0 percent greatly increases the rate of delignification. Here also, delignication is more rapid when 5 :percent water is added to the DMSO, -but in this case a decrease in the delignication rate takes place towards the end of the cook so that after 3 hours cooking time, almost identical amounts of residual lignin were found in pulps obtained from cooks lwith 5 percent water added or with no water added.

Similar conclusions may be drawn from a comparison of FIGURES 1 and 11.

(e) Delignification of aspen poplar sawdust with solutions of 5 percent SO2 in DMSO 1 5% water added to cooking liquor.

The results of a few cooks performed with aspen poplar sawdust are `shown in Table I. It may be seen that with 5 percent water added, delignitication is complete after 2 hours at 140 deg. C. Under the same conditions a black spruce pulp still contains 14.5 percent residual lignin. The -much greater ease of pulping of aspen sawdust may be due for the most part to the different chemical strucproduce such rapid and extensive delignification.

(f) Delgnificaton of aspen poplar chips with solutions of 5 percent SO2 iny DMSO Aspen chips which were impregnated with cooking liquor, first by removing the air in the chips under vacuum and then by soaking in the cooking liquor for one hour, were cooked by immersing the sealed containers in an oil bath at 140 deg. C. Maximum temperature was attained in ve minutes. No burning of the chips could be observed and delignification proceeded as smoothly as for aspen sawdust, as may be seen from FIGURE 13. It was observed that the chips could be defibered after only a half hour at 140 deg. C. and if water had been added to the cooking liquor, very easily after one hour of cooking. All pulps were light buff in colour but pulps obtained from the anhydrous cooks always had a darker colour.

FIGURE 12 illustrates the results of experiments conducted with solutions of 2% NO2 in DMSO.

As may be seen from FIGURE l2, the delignitiying action of a 2 percent NO2 solution in DMSO on black spruce sawdust is Very rapid. Even after 1 hour at 100 deg. C. appreciable amounts of lignin have been removed from the wood. With 5 percent water added, 1 hour at 140 deg. C. was sufficient to produce a pulp with only 4 percent lignin, in 48.8 percent yield. Thus NO2 solutions in DMSO are much more powerful delignifying agents than SO2 solutions of the same concentration. Nitration and oxidation of the lignin also takes place and the methoxyl content becomes lower than in the native lignin. The effect on the carbo-hydrate fraction, however, is surprisingly mild. This may ybe seen from the cupriethylenediamine viscosities of an NO2 pulp shown in Table III, which were somewhat higher than those of SO2 pulps. Also surprising is the small amount of NO2 required to NO2 cooks produce even faster deligniication of aspen wood.

FIGURES 14, 15, 16 and 17 illustrate the results of experiments using solution of 5% C12 in DMSO.

As may be seen from FIGURES 14 and 15, delignications of black spruce sawdust proceeded very quickly. Even after one hour cooking at deg. C. with 5 percent water added to the cooking liquor, the pulp yield was about 60 percent. After 1 hour at 125 deg. C. the pulp yield was down to about 41 percent. Here again, the addition of as little as 5 percent water to the cooking liquor greatly increased the rate of lignin removal.

The same ease of pulping was observed when spruce chips were used instead of sawdust. The chips were impregnated with cooking liquor prior to pulping, as described above.

The results obtained with aspen poplar are similar to those obtained with black spruce as may be seen from FIGURES 16 and 17. Delignication of aspen is easier than that of black spruce since the lignin content of the pulp has fallen to almost zero after 1 hour at 115 deg, C. It will be apparent that aspen is an appropriate example of a hardwood suitable for pulping whereas spruce is a good example of a softwood for this purpose.

It is observed that pulps obtained with C12 solutions in DMSO are dii-ferent from SO2 and NO2 pulps in that they are insoluble in cupriethylenediamine.

Copper numbers were determined :on pulps obtained with C12 in DMSO; these are listed in Table IV.

The lignins recovered from C12 cooks were slightly chlorinated and had lower methoxyl contents than the native lignin obtainable from the wood indicating that some oxidative demethoxylation had taken place.

Table II shows the alpha-cellulose content of black spruce and aspen poplar pulps obtained under different pulping conditions. Most pulps received only a short bleaching treatment before the alpha-cellulose determination, i.e., a half-hour treatment with NaClO2 at 75 deg. C.

TABLE II.-ALPHACELLULOSE IN PULPS FROM VARIOUS COOKS 1 5% water added to cooking liquor.

Of special interest is the very high alpha-cellulose content of pulps obtained from the percent C12 in DMSO cooks.

From Table III it may be seen that all pulps obtained by treating black spruce or aspen wood with SO2 or NO2 solutions in DMSO have cupriethylenediamine viscosities much higher than either a high alpha pulp or a standard bleached sulphite pulp. This clearly indicates that by using an organic solvent, such as DMSO, as cooking medium, much hydrolytic degradation of the cellulose is avoided. The short cooking times required for extensive delignication are also a factor in keeping cellulose degradation to a minimum.

TABLE III.-CUPRIETHYENEDIAMINE VISCOSITIES OF SO2 AND NO2 PULPS As may be seen from Table IV, the copper No. of two pulps from cooks with 5 percent C12 in DMSO is lower than that of a high alpha bleached sulphite pulp, which indicates that little cellulose degradation took place during the delignication.

TABLE IV.COPPER NO. OF PULPS FROM COOKS WITH 5% C12 IN DMSO Material Cooking conditions Cu No.

Black sprice chips 125 C., 1 hr., 5% water 0. 63

added. Aspen chips 115 C., 1 hr., 5% water 0. 40

added. High alpha bleached sul- 1.00

phite pulp (commercial).

It will be observed that solutions of SO2, NO2 and C12 in DMSO show excellent pulping properties with sawdust and chips. Using solutions of, for instance, 5% C12 and 2% NO2 in DMSO, pulps are obtained with low lignin and high alpha-cellulose contents. All pulps had cupriethylenediamine viscositie-s about twice as high as that of commercial bleached sulphite pulp. Moreover, the dissolved lignin can easily be precipitated from the cooking liquors by the addition of water. Pulping may be carried out at temperatures of 140 C. or lower, with cooking times ranging from 1/2 to 3 hours. It may be mentioned that all solutions of gas in DMSO are given as grams/ ml.

Conclusions may be summarized as follows:

(a) Dilute solutions of SO2, NO2 and C12 in DMSO produce extensive delignilication.

(b) Cooking times are short and temperatures need not exceed deg. C.; in most cases lower temperatures can be applied.

(c) By varying the cooking time or the temperature, pulps may be obtained ranging in yields from 80 percent to 42 percent.

(d) Pulps with high alpha-cellulose and high cupriethylenediamine viscosities or low copper numbers are easily obtained.

(e) Sawdust or chips may be pulped with equal ease.

(f) Pulps are easy to bleach.

(g) The dissolved lignin may be easily recovered from the cooking liquors by the simple addition of water.

(h) Since the addition of inorganic salts is unnecessary, recovery of chemicals would be facilitated.

We claim:

1. A process for pulping lignocellulosic material which comprises cooking -said material at a temperature not substantially exceeding 140 C. in a cooking liquor comprising dimethylsulphoxide having dissolved therein about 1% to 10% of an inorganic gas selected from the group consisting of sulphur dioxide, nitrogen dioxide, and chlorine.

2. A process for pulping lignocellulosic material which comprises cooking said material at a temperature not substantially exceeding 140 C. in a cooking liquor comprising dimethylsulphoxide having dissolved therein about 2% to 10% of sulphur dioxide.

3. A process for pulping lignocellulosic material which comprises cooking said material at a temperature not substantially exceeding 140 C. in a cooking liquor comprising dimethylsulphoxide having dissolved therein about 2% to 10% of nitrogen dioxide.

4. A process for pulping lignocellulosic material which comprises cooking said material at a temperature not substantially exceeding 140 C. in a cooking liquor comprising dimethylsulphoxide having dissolved therein about 2 to 10% of chlorine.

References Cited by the Examiner UNITED STATES PATENTS 2,022,664 12/ 1935 Groombridge 162-72 FOREIGN PATENTS 573,329 3/1959 Canada.

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. WOLK, Examiner. 

1. A PROCESS FOR PULPING LIGNOCELLULOSIC MATERIAL WHICH COMPRISES COOKING SAID MATERIAL AT A TEMPERATURE NOT SUBSTANTIALLY EXCEEDING 140*C. IN A COOKING LIQUOR COMPRISING DIMETHYLSULPHOXIDE HAVING DISSOLVED THEREIN ABOUT 1% TO 10% OF AN INORGANIC GAS SELECTED FROM THE GROUP CONSISTING OF SULPHUR DIOXIDE, NITROGEN DIOXIDE, AND CHLORINE. 